A thermosensitive recording material is typically formed by respectively grinding and dispersing an ordinary colorless or light-colored electron-donating leuco dye and an electron-accepting coloring agent such as a phenolic compound into fine particles, mixing both substances, adding an adhesive (binder), a filler such as a pigment, a sensitizer, and other lubricants and other auxiliary agents, and applying the obtained coating liquid to a sheet-like support such as paper, synthetic paper, or a plastic film. A thermosensitive recording material develops color due to an instantaneous chemical reaction caused by heating with a thermal head, a hot stamp, a thermal pen, a laser beam, or the like so as to obtain a recording image. Thermosensitive recording materials are widely used in facsimiles, computer terminal printers, automatic vending machines, measurement recorders, and the like. In recent years, there has been an increase in thermosensitive recording label applications in which the back surface of a label is subjected to adhesion processing. There is a long period of time until these POS labels or labels for process management agents are used after being subjected to adhesion processing. Therefore, components such as plasticizers, emulsifiers, low-molecular-weight oligomers, and surfactants contained in the adhesive agent applied to the back surface pass through the support and reach the thermosensitive recording surface at the time of storage. This causes the inhibition of a color-developing reaction (glue desensitization) and leads to problems in that the recording performance in terms of recording color developability or recording image quality is dramatically reduced or in that printing voids occur.
In order to solve the problems described above, it has been proposed to provide a barrier layer containing a polyvinyl alcohol with a high degree of saponification and a pigment between the support and the adhesive layer (see Patent Document 1); to provide a back coat layer containing a styrene-butadiene copolymer and a pigment on the back surface of the support (see Patent Document 2); to provide a back surface layer containing a water-soluble acrylic resin as a main component on the surface of the support on the side opposite to the thermosensitive recording layer (see Patent Document 3); and to add a styrene-acrylic-based resin having a glass transition point of at most 10° C. and a permeability of at most 2.0% at 280 nm in a 0.01 wt. % aqueous solution to the back coat layer (see Patent Document 4). However, satisfactory results have not yet necessarily been achieved.
In addition, an adhesive label for thermosensitive recording in which an adhesive undercoat layer containing a filler primarily composed of a thermoplastic resin between the back coat layer on the back surface of the support and the adhesive layer has also been proposed (see Patent Document 5). Furthermore, it has also been proposed to use a urea urethane compound such as 4,4′-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenyl sulfone as a coloring agent (see Patent Document 6). However, these solutions have problems in that the inhibitory effect on poor coloring due to the adhesive component is insufficient or in that the production process becomes complex and the production efficiency decreases in order to achieve a sufficient effect.
In addition, woodfree paper is typically used as the support of a thermosensitive recording material. In the case of acidic paper making, paper is produced by adding a rosin-based sizing agent and a clay, talc, or the like as a filler. It is typical to use a sulfate band (aluminum sulfate) as a fixing agent of this rosin-based sizing agent, but the pH of the paper surface tends toward the acidic side due to residual sulfate radicals (sulfate ions) remaining in the paper. Therefore, the color-developing substance constituting the thermosensitive recording paper causes a reaction with acidic ions on the surface of the paper, which leads to a problem in that background fogging tends to occur during a long-term storage period. Neutral paper containing a basic pigment (alkaline filler) such as calcium carbonate may therefore be used as the support of a thermosensitive recording material for the purpose of preventing background fogging or reducing the paper making cost.
However, when a support containing a basic pigment is used as the support of the thermosensitive recording material, there are problems in that while the thermosensitive recording material is being stored, the coloring performance is diminished prior to recording, whereas printing is blurred and unclear or, in some cases, almost completely invisible due to discoloration after printing. In particular, when the color developability is diminished prior to recording, the printing density of the thermosensitive recording material is diminished, which makes the content difficult to decipher, dramatically diminishes the recording performance in terms of recording color developability and recording image quality that the material demonstrated prior to storage, and loses the original function as a thermosensitive recording material. The reason that the color developability is diminished is not clear, but is presumed to be due to the fact that the coloring agent forms a salt with the basic pigment (alkaline filler) contained in the support and causes a morphological change, which causes the performance of the coloring agent to be diminished.
In order to solve the problems described above, it has been proposed to use an alkyl ketene dimer as a synthetic sizing agent and to use neutral paper having a zeta potential of at most +20 mV in a solution of the synthetic sizing agent with a solid content concentration of 0.02% at a pH of 8.0 as a support (see Patent Document 7); and to provide a thermosensitive recording layer containing an alkali salt of a diosobutylene-maleic anhydride copolymer on neutral paper using an alkyl ketene dimer as a sizing agent (see Patent Document 8). However, satisfactory results have not yet necessarily been achieved.